In view of the popularity of four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for selectively directing power (i.e., drive torque) from the powertrain to all four wheels. In many power transfer systems, a transfer case is incorporated into the driveline and is operable in a four-wheel drive mode for delivering drive torque from the powertrain to both the front and rear wheels. Many conventional transfer cases are equipped with a mode shift mechanism that can be selectively actuated to shift between a two-wheel drive mode and a four-wheel drive mode. In addition, many transfer cases also include a range shift mechanism which can be selectively actuated by the vehicle operator for shifting between four-wheel high-range and low-range drive modes.
It is also known to use “on-demand” power transfer systems for automatically biasing power between the front and rear wheels, without any input or action on the part of the vehicle operator, when traction is lost at either the front or rear wheels. Modernly, it is known to incorporate the “on-demand” feature into a transfer case by replacing the mechanically-actuated mode shift mechanism with a clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is typically maintained in a non-actuated condition such that drive torque is only delivered to the rear wheels. However, when the sensors detect a low traction condition, the clutch assembly is automatically actuated to deliver drive torque to the front wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-slipping wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. This on-demand clutch control system can also be used in full-time transfer cases to automatically bias the torque ratio across an interaxle differential.
In some two-speed on-demand transfer cases, the range shift mechanism and the clutch assembly are independently controlled by separate power-operated actuators. For example, U.S. Pat. No. 5,407,024 discloses a two-speed range shift mechanism actuated by an electric motor and a clutch assembly actuated by an electromagnetic coil. In an effort to reduce cost and complexity, some transfer cases are equipped with a single power-operated actuator that is operable to coordinate actuation of both the range shift mechanism and the clutch assembly. In particular, U.S. Pat. Nos. 5,363,938 and 5,655,986 each illustrate a transfer case equipped with a motor-driven sector having cam surfaces adapted to coordinate actuation of the range shift mechanism and the clutch assembly for establishing a plurality of distinct two-wheel and four-wheel drive modes. Other examples of transfer cases with coordinated range and mode shift systems are shown in U.S. Pat. Nos. 6,645,109, 6,783,475 and 6,802,794. While some transfer cases equipped with coordinated clutch actuation systems have proven to be commercially successful, a need exists to develop alternative systems which further advance the art related to two-speed on-demand transfer cases.